A systematic study of phase equilibria of the Y2O3 – BaO – Fe2O3 system in the vicinity of YBa2Fe3O8+δ reveals that the actual composition of the 123-type oxide differs from the ideal one. It is shown that a single-phase oxide with a 123-type structure can be obtained at 1373 ​K in air only with a slight deficit of Ba and an excess of both Y and Fe components. The overall formula of the single-phase 123-type oxide is written as Y1.05Ba1.92Fe3.03O8+δ. The composition of one more complex oxide, which is absent in the reported earlier phase diagram, is also found to be Ba-deficient; it can be presented as Y1.04Ba0.96FeO4. As a result, the revised version of phase diagram for the Y2O3 – BaO – Fe2O3 system is constructed at 1373 ​K in air.